Lithium secondary batteries containing lithium salt-ionic liquid solvent electrolyte

ABSTRACT

A rechargeable lithium metal or lithium-ion cell comprising a cathode having a cathode active material and/or a conductive supporting structure, an anode having an anode active material and/or a conductive supporting nano-structure, a porous separator electronically separating the anode and the cathode, a highly concentrated electrolyte in contact with the cathode active material and the anode active material, wherein the electrolyte contains a lithium salt dissolved in an ionic liquid solvent with a concentration greater than 3 M. The cell exhibits an exceptionally high specific energy, a relatively high power density, a long cycle life, and high safety with no flammability.

FIELD OF THE INVENTION

The present invention provides an ionic liquid-based electrolyte composition and a secondary or rechargeable lithium metal or lithium-ion battery containing such an electrolyte composition.

BACKGROUND

Rechargeable lithium-ion (Li-ion), lithium metal, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li_(4.4)Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries have a significantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having relatively high specific capacities, such as TiS₂, MoS₂, MnO₂, CoO₂, and V₂O₅, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte and the cathode became lithiated. Unfortunately, upon repeated charges and discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately caused internal shorting and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.

Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries (e.g. Lithium-sulfur and Lithium-transition metal oxide cells) for EV, HEV, and microelectronic device applications. Again, cycling stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li metal to form dendrite structures during repeated charge-discharge cycles or overcharges, leading to internal electrical shorting and thermal runaway. This thermal runaway or even explosion has a lot to do with the organic solvents used in the electrolyte (e.g. carbonate and ether families of solvents), which are unfortunately highly volatile and flammable.

Many attempts have been made to address the dendrite and thermal runaway issues. However, despite these earlier efforts, no rechargeable Li metal batteries have succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures designed for prevention of dendrites are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. In most of the lithium metal cells and lithium-ion cells, the electrolyte solvents are flammable. An urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries and other rechargeable batteries.

Parallel to these efforts and prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as Li_(x)C₆, where x is typically less than 1.

Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li⁺ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-170 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range of 120-220 Wh/kg, most typically 150-180 Wh/kg. These specific energy values are two to three times lower than what would be required for battery-powered electric vehicles to be widely accepted.

Furthermore, the same flammable solvents are used in most of the lithium-ion batteries. Despite the notion that there is significantly reduced propensity of forming dendrites in a lithium-ion cell (relative to a lithium metal cell), the lithium-ion cell has its own intrinsic safety issue. For instance, the transition metal elements in the lithium metal oxide cathode are highly active catalysts that can promote and accelerate the decomposition of organic solvents, causing thermal runaway or explosion initiation to occur at a relatively low electrolyte temperature (e.g. <200° C., as opposed to normally 400° C. without the catalytic effect).

Ionic liquids (ILs) are a new class of purely ionic, salt-like materials that are liquid at unusually low temperatures. The official definition of ILs uses the boiling point of water as a point of reference: “Ionic liquids are ionic compounds which are liquid below 100“C”. A particularly useful and scientifically interesting class of ILs is the room temperature ionic liquid (RTIL), which refers to the salts that are liquid at room temperature or below. RTILs are also referred to as organic liquid salts or organic molten salts. An accepted definition of an RTIL is any salt that has a melting temperature lower than ambient temperature.

Although ILs were suggested as a potential electrolyte for rechargeable lithium batteries due to their non-flammability, conventional ionic liquid compositions have not exhibited satisfactory performance when used as an electrolyte likely due to several inherent drawbacks: (a) ILs have relatively high viscosity at room or lower temperatures; thus being considered as not amenable to lithium ion transport; (b) For Li—S cell uses, ILs are capable of dissolving lithium polysulfides at the cathode and allowing the dissolved species to migrate to the anode (i.e., the shuttle effect remains severe); and (c) For lithium metal secondary cells, most of the ILs strongly react with lithium metal at the anode, continuing to consume Li and deplete the electrolyte itself during repeated charges and discharges. These factors lead to relatively poor specific capacity (particularly under high current or high charge/discharge rate conditions, hence lower power density), low specific energy density, rapid capacity decay and poor cycle life. Consequently, as of today, no commercially available lithium battery makes use of an ionic liquid as the primary electrolyte component. This is quite unfortunate considering the fact that ILs can provide non-flammable and safe batteries and should have been widely implemented in rechargeable batteries.

With the rapid development of hybrid (REV), plug-in hybrid electric vehicles (HEV), and all-battery electric vehicles (EV), there is an urgent need for anode and cathode materials and electrolytes that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. One of the most promising energy storage devices is the lithium-sulfur (Li—S) cell since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode. The lithium-sulfur cell operates with a redox couple, described by the reaction S₈+16Li→8Li₂S that lies near 2.2 V with respect to Li⁺/Li^(o). This electrochemical potential is approximately 2/3 of that exhibited by conventional positive electrodes. However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Values can approach 2,500 Wh/kg or 2,800 Wh/l based on the combined Li and S weight or volume (not based on the total cell weight or volume), respectively, assuming complete reaction to Li₂S. With a proper cell design, a cell-level specific energy of 1,200 Wh/kg (of cell weight) and cell-level energy density of 1,400 Wh/l (of cell volume) should be achievable. However, the current Li-sulfur products of industry leaders in sulfur cathode technology have a maximum cell specific energy of 400 Wh/kg (based on the total cell weight), far less than what could be obtained in real practice.

In summary, despite its considerable advantages, the rechargeable lithium metal cell in general and the Li—S cell in particular is plagued with several major technical problems that have hindered its widespread commercialization:

-   (1) Conventional lithium metal secondary cells (e.g., rechargeable     Li metal cells, Li—S cells, Li-Air cells) still have dendrite     formation and related internal shorting and thermal runaway issues.     Also, conventional Li-ion cells still make use of significant     amounts of flammable liquids (e.g. propylene carbonate, ethylene     carbonate, etc) as the primary electrolyte solvent, risking danger     of explosion; -   (2) For the Li—S cells, sulfur or sulfur-containing organic     compounds are highly insulating, both electrically and ionically. To     enable a reversible electrochemical reaction at high current rates,     the sulfur must maintain intimate contact with an electrically     conductive additive. Various carbon-sulfur composites have been     utilized for this purpose, but only with limited success owing to     the scale of the contact area. The use of large amounts of inactive     materials (conductive additives) also implies a poor cathode active     material utilization rate. Typical reported capacities are between     300 and 550 mAh/g (based on carbon-sulfur composite weight) at     moderate rates. -   (3) Other cathode active materials, such as TiS₂, MoS₂, MnO₂, CoO₂,     iron oxide, and various types of vanadium oxide (V₂O₅, VO₂, V₃O₈,     etc), are also electrically non-conducting and, hence, the resulting     lithium metal cells suffer from the same poor rate capability and     low active material utilization efficiency as the Li—S cell. -   (4) The Li—S cell tends to exhibit significant capacity degradation     during discharge-charge cycling. This is mainly due to the high     solubility of the lithium polysulfide anions formed as reaction     intermediates during both discharge and charge processes in the     polar organic solvents used in electrolytes. During cycling, the     lithium polysulfide anions can migrate through the separator and     electrolyte to the Li negative electrode whereupon they are reduced     to solid precipitates (Li₂S₂ and/or Li₂S), causing active mass loss.     In addition, the solid product that precipitates on the surface of     the positive electrode during discharge becomes electrochemically     irreversible, which also contributes to active mass loss. -   (5) More generally speaking, a significant drawback with cells     containing cathodes comprising elemental sulfur, organosulfur and     carbon-sulfur materials relates to the dissolution and excessive     out-diffusion of soluble sulfides, polysulfides, organo-sulfides,     carbon-sulfides and/or carbon-polysulfides (hereinafter referred to     as anionic reduction products) from the cathode into the rest of the     cell. This phenomenon is commonly referred to as the Shuttle Effect.     This process leads to several problems: high self-discharge rates,     loss of cathode capacity, corrosion of current collectors and     electrical leads leading to loss of electrical contact to active     cell components, fouling of the anode surface giving rise to     malfunction of the anode, and clogging of the pores in the cell     membrane separator which leads to loss of ion transport and large     increases in internal resistance in the cell.

In response to these challenges, new electrolytes, protective films for the lithium anode, and solid electrolytes have been developed. Some interesting cathode developments have been reported recently to contain lithium polysulfides; but, their performance still fall short of what is required for practical applications. Despite the various approaches proposed for the fabrication of high energy density rechargeable cells containing elemental sulfur, organo-sulfur and carbon-sulfur cathode materials, or derivatives and combinations thereof, there remains a need for materials and cell designs that (a) retard the out-diffusion of anionic reduction products, from the cathode compartments into other components in these cells, (b) improve the utilization of electro-active cathode materials and the cell efficiencies, and (c) provide rechargeable cells with high capacities over a large number of cycles.

Again, lithium metal (including pure lithium, alloys of lithium with other metal elements, or lithium-containing compounds) still provides the highest anode specific capacity as compared to essentially all other anode active materials (except pure silicon, but silicon has pulverization issues). Lithium metal would be an ideal anode material in a lithium-sulfur secondary battery if dendrite related issues could be addressed. In addition, there are several non-lithium anode active materials that exhibit high specific lithium-storing capacities (e.g., Si, Sn, SnO₂, and Ge as an anode active material) in a lithium ion battery wherein lithium is inserted into the lattice sites of Si, Sn, SnO₂, or Ge in a charged state. These have been largely ignored in the prior art Li—S cells.

Hence, a general object of the present invention is to provide an electrolyte system for a rechargeable lithium cell that exhibits a high energy density, high power density, long cycle life, and no danger of explosion (due to the use of a safer, non-flammable ionic liquid-based electrolyte solvent). The invention also provides a rechargeable lithium cell containing such a safe electrolyte system. This lithium cell includes the lithium metal cell (e.g. Li—S, Li—TiS₂, Li—MoS₂, Li—VO₂, and Li-air, just to name a few), lithium-ion cell (e.g. graphite-LiMn₂O₄, Si—Li_(x)Ni_(y)Mn_(z)O₂, etc), Li-ion sulfur cell (e.g. prelithiated Si—S cell), and hybrid lithium cell (wherein at least one electrode operates on lithium insertion or intercalation).

A specific object of the present invention is to provide a rechargeable Li—S battery that exhibits an exceptionally high specific energy or high energy density and a high level of safety. One specific technical goal of the present invention is to provide a safe Li metal-sulfur or Li ion-sulfur cell with a cell specific energy greater than 500 Wh/Kg, preferably greater than 600 Wh/Kg, and more preferably greater than 800 Wh/Kg (all based on the total cell weight).

Another specific object of the present invention is to provide a safe lithium-sulfur cell that exhibits a high specific capacity (higher than 1,200 mAh/g based on the sulfur weight, or higher than 1,000 mAh/g based on the cathode composite weight, including sulfur, conducting additive and conductive substrate, and binder weights combined, but excluding the weight of cathode current collector). The specific capacity is preferably higher than 1,400 mAh/g based on the sulfur weight alone or higher than 1,200 mAh/g based on the cathode composite weight. This must be accompanied by a high specific energy, good resistance to dendrite formation, good resistance to thermal runaway, no possibility of an explosion, and a long and stable cycle life.

It may be noted that in most of the open literature reports (scientific papers) on Li—S cells, scientists choose to express the cathode specific capacity based on the sulfur weight or lithium polysulfide weight alone (not on the total cathode composite weight), but unfortunately a large proportion of non-active materials (those not capable of storing lithium, such as conductive additive and binder) is typically used in their Li—S cells. Similarly, for lithium-vanadium oxide cells, scientists also tend to report the cathode specific capacity based on the vanadium oxide weight only. For practical usage purposes, it is more meaningful to use the cathode composite weight-based capacity value.

A specific object of the present invention is to provide a rechargeable lithium-sulfur cell based on rational materials and battery designs that overcome or significantly reduce the following issues commonly associated with conventional Li—S cells: (a) dendrite formation (internal shorting); (b) extremely low electric and ionic conductivities of sulfur, requiring large proportion (typically 30-55%) of non-active conductive fillers and having significant proportion of non-accessible or non-reachable sulfur or lithium polysulfides); (c) dissolution of lithium polysulfide in electrolyte and migration of dissolved lithium polysulfides from the cathode to the anode (which irreversibly react with lithium at the anode), resulting in active material loss and capacity decay (the shuttle effect); and (d) short cycle life.

Another object of the present invention is to provide a simple, cost-effective, and easy-to-implement approach to preventing potential Li metal dendrite-induced internal short circuit and thermal runaway problems in various Li metal and Li-ion batteries.

SUMMARY OF THE INVENTION

The present invention provides a rechargeable lithium battery (including various types of lithium metal or lithium-ion cells) featuring a non-flammable, safe, and high-performing electrolyte. The cell comprises a cathode having a cathode active material and/or a cathode conductive supporting structure, an anode having an anode active material and/or a conductive supporting nano-structure, a separator electronically separating the anode and the cathode, a highly concentrated electrolyte in contact with the cathode active material (or the cathode conductive supporting structure for a Li-air cell) and the anode active material, wherein the electrolyte contains a lithium salt dissolved in a first ionic liquid solvent with a salt concentration greater than 3 M (mole/liter). The concentration is preferably greater than 4 M, more preferably greater than 5 M, further more preferably greater than 7 M, and most preferably greater than 10 M.

As indicated earlier, one of the major problems inherently associated with using ILs as a battery electrolyte ingredient is the notion that ILs have relatively high viscosity at room or lower temperatures and the high viscosity is considered as not amenable to fast lithium ion transport. One would expect that the addition of a higher amount of lithium salt in an IL (or any liquid) would make the liquid even more viscous and less suitable to lithium ion diffusion. We have defied these expectations by discovering that higher concentrations of lithium salts in an ionic liquid actually provide many unexpected benefits to a lithium secondary cell, including more facile lithium ion transport, reduced or eliminated shuttle effect (in a Li—S cell), reduced or eliminated irreversible Li-electrolyte reactions (hence, improved Li utilization efficiency), high specific energy density, high power density, long and stable cycling life, and high level of safety.

In one embodiment, the first ionic liquid solvent in the invented battery is a room temperature ionic liquid (RTIL). The first ionic liquid solvent may be preferably selected from a room temperature ionic liquid having a cation selected from tetraalkylammonium, di-, tri-, or tetra-alkylimidazolium, alkylpyridinium, dialkylpyrrolidinium, dialkylpiperidinium, tetraalkyl-phosphonium, trialkylsulfonium, or a combination thereof. In another embodiment, the first ionic liquid solvent may be selected from a room temperature ionic liquid having an anion selected from BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, or a combination thereof.

In one preferred embodiment, the electrolyte further contains a solubility-promoting agent, which enables the first ionic liquid solvent to dissolve more lithium salt than it can without such a solubility-promoting agent. Typically, a lithium salt can be dissolved in an electrolyte solvent (particularly ionic liquid solvent) up to a concentration less than 2 M, more typically less than 1.5 M, and most typically between 0.3 M and 1.0 M. Quite unexpectedly, the addition of a judicially selected solubility-promoting agent can significantly increase the solubility limit to typically greater than 5 M and, in several cases, greater than 10 M.

The amount of the solubility-promoting agent is an important consideration. In one embodiment, the ratio of the solubility-promoting agent amount to the first ionic liquid solvent amount is less than 1/3, preferably less than 1/4, further preferably less than 1/5, more preferably less than 1/7, still more preferably less than 1/9, and most preferably less than 1/10. Depending upon the compositions, the electrolyte appears to remain non-flammable when the ratio of the solubility-promoting agent amount to the first ionic liquid solvent amount is less than 1/3 or less than 1/4. In other words, the resulting mixture solvents (particularly when in the presence of a lithium salt) remain non-flammable, non-volatile, and safe if the flammable organic solvent-based solubility-promoting agent does not exceed 20%-25% of the mixture solvent (not counting the salt dissolved therein). This is a critically important consideration when it comes to complete elimination of explosion danger of an EV battery.

The solubility-promoting agent may be selected from an organic liquid or a second ionic liquid, provided it can significantly increase the solubility limit of a lithium salt in the first ionic liquid solvent. The ionic liquid-based electrolyte may contain an ether-based solubility-promoting agent selected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), Poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, a combination thereof, or a combination with a carbonate.

The solubility-promoting agent in the ionic liquid-based electrolyte may be selected from carbonate-based organic material, such as ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene or methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), or a combination thereof.

The lithium salt may be selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), an ionic liquid lithium salt, or a combination thereof.

It is of significance to note that typically any of the aforementioned lithium salts can be dissolved in an ionic liquid solvent alone for up to a salt concentration of approximately 1.5 M. Also typically, the same lithium salt can be dissolved in an organic solvent for up to a concentration of 2-3 M only. We have unexpectedly discovered that mixtures of ionic liquid solvent-organic solvent (with an IL-to-organic ratio of from 3:1 to >10:1) can typically dissolve the same lithium salt up to 5-10 M (or even >10 M in several cases). Once, complete dissolution occurs, the lithium salt can remain dissolved without crystallization or precipitation even after the solubility-promoting agent is removed. This is a very significant discovery by itself since this provides an added dimension in selecting electrolytes for various lithium rechargeable batteries.

Even more significant and scientifically most surprising is the further discovery that such a high concentration of lithium salt added to the intrinsically highly viscous ionic liquid results in a quasi-solid state electrolyte that enables facile transport of lithium ions therein. Such a quasi-solid electrolyte has the best properties of both liquid electrolytes and solid electrolytes. Such a quasi-solid electrolyte, like a solid state electrolyte, prevents dendrite penetration in a lithium metal cell and prevents lithium polysulfide dissolution in the cathode of a Li—S cell. However, it does not have the issues of poor electrode wettability, large electrolyte-electrode interfacial resistance, and extremely low lithium ion diffusivity commonly associated with solid electrolytes. On the contrary, this quasi-solid electrolyte wets both the anode and cathode electrodes very well, conducts lithium ions very well (likely through a totally different conduction mechanism), does not impart any significant interfacial resistance. This quasi-solid electrolyte has the best of both liquid and solid electrolyte worlds.

In the presently invented rechargeable lithium metal or lithium-ion cell, the anode contains an anode active material selected from lithium metal, a lithium metal alloy, a mixture of lithium metal or lithium alloy with a lithium intercalation compound, a lithiated compound, lithiated titanium dioxide, lithium titanate, lithium manganate, a lithium transition metal oxide, Li₄Ti₅O₁₂, or a combination thereof.

In one embodiment, the rechargeable lithium cell is a lithium-sulfur cell that further comprises a layer of protective material disposed between the anode and the electrolyte wherein the protective material is a lithium ion conductor. The protective material may consist of a solid electrolyte.

In one embodiment, the rechargeable lithium metal or lithium-ion cell is a Lithium-air cell (or Li—O₂ cell) having a higher round-trip efficiency or higher resistance to capacity decay as compared to a corresponding Lithium-air cell that has an electrolyte salt concentration lower than 3M. Preferably, an exfoliated graphite worm or graphene-based meso-porous structure serves as a conductive supporting structure.

In an embodiment, the cathode may comprise a cathode active material consisting of (a) exfoliated graphite worms that are interconnected to form a porous, conductive graphite flake network comprising pores having a size smaller than 100 nm; and (b) nano-scaled powder or coating of sulfur, sulfur compound, or lithium polysulfide disposed in the pores or coated on a graphite flake surface wherein the powder or coating is in contact with the electrolyte and has a dimension less than 100 nm (preferably less than 20 nm, more preferably <10 nm, and most preferably <5 nm). Preferably, the exfoliated graphite worm amount is in the range of 1% to 90% by weight and the amount of powder or coating is in the range of 99% to 10% by weight based on the total weight of exfoliated graphite worms and sulfur, sulfur compound, or lithium polysulfide combined, which is measured or calculated when the cell is in a fully charged state.

Alternatively, the cathode active material (e.g. sulfur or lithium polysulfide in a nano-scaled particle or coating form) may be supported by a nano-structure of electrically conductive nanometer-scaled filaments that are interconnected to form a porous network of electron-conducting paths comprising interconnected pores, wherein the filaments (e.g. carbon nanotubes or graphene sheets) have a transverse dimension (diameter or thickness) less than 500 nm and wherein the nano-structure provides a sufficient amount of nano-filament surface areas to support the cathode active material (e.g. lithium polysulfide) when the cell is in a discharged state.

In one embodiment, the lithium-sulfur cell is a lithium metal-sulfur cell containing a lithium metal, lithium metal alloy, or lithium compound as an anode active material. In another embodiment, the lithium-sulfur cell is a lithium ion-sulfur cell wherein the anode contains an anode active material selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd), and lithiated versions thereof; (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiated versions thereof, wherein said alloys or compounds are stoichiometric or non-stoichiometric; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, and lithiated versions thereof; (d) salts and hydroxides of Sn and lithiated versions thereof; (e) carbon or graphite materials and prelithiated versions thereof; and combinations thereof.

The carbon or graphite materials may be selected from the group consisting of natural graphite particles, synthetic graphite particles, needle cokes, electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, carbon nanowires, sheets and platelets of pristine graphene, graphene oxide, reduced graphene oxide, doped graphene or graphene oxide, and chemically functionalized graphene, and combinations thereof.

The lithium-sulfur cell may further comprise a cathode current collector selected from aluminum foil, carbon- or graphene-coated aluminum foil, stainless steel foil or web, carbon- or graphene-coated steel foil or web, carbon or graphite paper, carbon or graphite fiber fabric, flexible graphite foil, graphene paper or film, or a combination thereof. A web means a screen-like structure or a metal foam, preferably having interconnected pores or through-thickness apertures. The lithium-sulfur cell may further comprise an anode current collector selected from copper foil or web, carbon- or graphene-coated copper foil or web, stainless steel foil or web, carbon- or graphene-coated steel foil or web, titanium foil or web, carbon- or graphene-coated titanium foil or web carbon or graphite paper, carbon or graphite fiber fabric, flexible graphite foil, graphene paper or film, or a combination thereof.

The presently invented lithium-sulfur cell provides a reversible specific capacity of typically no less than 800 mAh per gram based on the total weight of exfoliated graphite worms and sulfur (or sulfur compound or lithium polysulfide) combined. More typically and preferably, the reversible specific capacity is no less than 1,000 mAh per gram and often exceeds 1,200 mAh per gram. The high specific capacity of the presently invented cathode, when in combination with a lithium anode, leads to a cell specific energy of no less than 600 Wh/Kg based on the total cell weight including anode, cathode, electrolyte, separator, and current collector weights combined. In many cases, the cell specific energy is higher than 800 Wh/Kg and, in some examples, exceeds 1,000 Wh/kg.

The rechargeable lithium cell of the present invention featuring a highly concentrated ionic liquid-based electrolyte is not limited to lithium metal-sulfur cell or lithium-ion cell. This safe and high-performing electrolyte can be used in any lithium metal secondary cell (lithium metal-based anode coupled with any cathode active material) and any lithium-ion cell. The lithium metal or lithium-ion cell may contain a cathode active material selected from a transition metal dichalcogenide (e.g., TiS₂, TaS₂, and MoS₂), a transition metal trichalcogenide (e.g., NbSe₃), a transition metal oxide (e.g., MnO₂, CoO₂, an iron oxide, a vanadium oxide, etc), or a combination thereof.

The cathode active material may contain a vanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂, V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.

Broadly speaking, but still within the scope of the present invention, the rechargeable lithium metal or lithium-ion cell may contain a cathode active material selected from a layered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

These and other advantages and features of the present invention will become more transparent with the description of the following best mode practice and illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 SEM images of exfoliated graphite worms: (A) a graphite worm imaged at a low magnification; (B) same graphite worm as in (A), but taken at a higher magnification.

FIG. 2 SEM images of sulfur melt-impregnated exfoliated graphite worms: (A) an impregnated graphite worm imaged at a low magnification; (B) same impregnated graphite worm as in (A), but taken at a higher magnification.

FIG. 3 (A) Cycling performance (charge specific capacity, discharge specific capacity, and Coulomb efficiency) of a Li metal-sulfur cell containing a low-concentration electrolyte (1 M of Li salt in an IL solvent) and (B) representative charge-discharge curves of the same cell.

FIG. 4 (A) Cycling performance (charge specific capacity, discharge specific capacity, and Coulomb efficiency) of a Li metal-sulfur cell containing a high-concentration IL electrolyte (C=3.5 M); (B) behaviors of a cell with higher-concentration electrolyte (5.5 M).

FIG. 5 Ragone plots (cell power density vs. cell energy density) of three Li metal-sulfur cells each having an exfoliated graphite worm-sulfur cathode, but the lithium salt concentrations being 1.0 M, 3.5 M, and 5.5 M, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a rechargeable lithium battery (including various types of lithium metal or lithium-ion cells) featuring a non-flammable, safe, and high-performing electrolyte composed of a large proportion of lithium salt dissolved in an ionic liquid. In one embodiment, the cell comprises a cathode having a cathode active material, an anode having an anode active material, a porous separator electronically separating the anode and the cathode, a highly concentrated electrolyte in contact with the cathode active material and the anode active material, wherein the electrolyte contains a lithium salt dissolved in a first ionic liquid solvent with a salt concentration greater than 3 M (mole/liter). The concentration is preferably greater than 4 M, more preferably greater than 5 M, further more preferably greater than 7 M, and most preferably greater than 10 M.

A wide range of lithium salts may be used in the presently invented rechargeable lithium cell. Preferred lithium salts include lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), and any ionic liquid salt containing lithium as a cation (this ionic liquid salt is used as a solute, not as a solvent).

The ionic liquid is composed of ions only. Ionic liquids are low melting temperature salts that are in a molten or liquid state when above a desired temperature. For instance, an ionic salt is considered as an ionic liquid if its melting point is below 100° C. If the melting temperature is equal to or lower than room temperature (25° C.), the salt is referred to as a room temperature ionic liquid (RTIL). The IL salts are characterized by weak interactions, due to the combination of a large cation and a charge-delocalized anion. This results in a low tendency to crystallize due to flexibility (anion) and asymmetry (cation).

A well-known ionic liquid is formed by the combination of a 1-ethyl-3-methyl-imidazolium (EMI) cation and an N,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combination gives a fluid with an ionic conductivity comparable to many organic electrolyte solutions, a low decomposition propensity and low vapor pressure up to ˜300-400° C. This implies a generally low volatility and non-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic or inorganic ions that come in an unlimited number of structural variations owing to the preparation ease of a large variety of their components. Thus, various kinds of salts can be used to design the ionic liquid that has the desired properties for a given application. These include, among others, imidazolium, pyrrolidinium and quaternary ammonium salts as cations and bis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide and hexafluorophosphate as anions. Based on their compositions, ionic liquids come in different classes that include three basic types: aprotic, protic and zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, but are not limited to, tetraalkylammonium, di, tri, and tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILs include, but are not limited to, BF₄ ⁻, B(CN)₄ ⁻, CH3BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc. Relatively speaking, the combination of imidazolium- or sulfonium-based cations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs with good working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionic conductivity, high thermal stability, low volatility, low (practically zero) vapor pressure, non-flammability, the ability to remain liquid at a wide range of temperatures above and below room temperature, high polarity, high viscosity, and wide electrochemical windows. These properties, except for the high viscosity, are desirable attributes when it comes to using an RTIL as an electrolyte solvent in a rechargeable lithium cell.

At room or lower temperatures, ILs have relatively high viscosity, which is generally thought to be unsuitable for lithium ion transport. One would expect that the addition of a higher amount of lithium salt in an IL would make the liquid even more viscous and even less suitable to lithium ion diffusion. Contrary to these expectations, we have discovered that higher concentrations of lithium salts in an ionic liquid (>>3 M) actually provide many surprising benefits to a lithium secondary cell, including a more facile lithium ion transport, reduced or eliminated shuttle effect (in a Li—S cell), reduced or eliminated irreversible Li-electrolyte reactions (hence, improved Li utilization efficiency), high specific energy density, high power density, long and stable cycling life, and high level of safety.

When an ionic liquid alone is used as a solvent to dissolve lithium salt therein, the resulting electrolyte is non-flammable and safe. When a small amount of organic solvent as a solubility-promoting agent is mixed with an ionic liquid, the resulting electrolyte is also non-flammable provided the lithium salt concentration exceeds 3 M and the IL-to-organic solvent ratio is at least 3/1 (i.e. the organic solvent is less than 25%).

After an extensive and in-depth research effort, we have come to realize that such an ionic liquid solvent-based high concentration electrolyte system surprisingly solves the most critical issues associated with current lithium metal secondary batteries. This is further described as follows:

-   -   (a) The use of a high-concentration electrolyte at the anode         side appears to have significantly reduced or eliminated any         lithium dendrite-like feature.     -   (b) Also quite significantly, this electrolyte provides a much         higher lithium utilization rate at the anode. In most, if not         all, of the conventional rechargeable lithium metal cells         (including all Li—S cells), the electrolyte typically contains a         concentration of 0.5-2 M of lithium salt in a solvent, implying         the presence of a relatively large quantity of liquid solvent.         During charge-discharge cycles, the lithium metal at the anode         continues to react with liquid electrolyte solvent in an         irreversible manner, thus, continuing to consume the electrolyte         and lithium. This has been a significant problem to which no         effective solution has been found. It seems that the use of a         high-concentration electrolyte has provided an environment not         conducive to the occurrence of such irreversible reactions.     -   (c) At the cathode side, for a Li—S cell, the higher salt         concentration electrolyte is also beneficial in that the ionic         solvent alone (or with a small amount of an organic solvent) has         already dissolved a lithium salt practically up to or exceeding         its solubility limit, leaving no or little room for any         additional salt. This implies that the lithium polysulfide that         forms at the cathode during discharge of a Li—S cell will no         longer be dissolved in the already highly saturated solvent.         Furthermore, the high viscosity of the ionic liquid electrolyte         would also mean reduced possibility of having dissolved lithium         polysulfide species migrating to the anode, where they         irreversibly react with lithium to form species that prevent         sulfide from returning back to the cathode during the subsequent         discharge operation of the Li—S cell (the detrimental shuttling         effect). It seems that the presence of a high-concentration         electrolyte reduces the dissolution of lithium polysulfide and         also forces most of the dissolved lithium polysulfide (if any)         to substantially stay at the cathode side.     -   (d) The ionic liquid-based electrolyte, even at a high salt         concentration, does not have any difficulty in wetting various         cathode active materials, including sulfur, transition metal         dichalcogenides, transition metal trichalcogenides, transition         metal oxides, etc. Actually, the good wettability of the ionic         liquid-based high-concentration electrolyte has enabled a better         cathode active material utilization rate (particularly in         rechargeable Li—S cells and Li metal cells containing TiS₂,         TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂, iron oxide, or vanadium oxide as         a cathode active material.

The above observations have demonstrated the many unexpected synergistic and beneficial effects of using an ionic liquid solvent-based electrolyte system having a high lithium salt concentration in a lithium-sulfur secondary cell.

In one preferred embodiment, the electrolyte further contains a solubility-promoting agent, which enables the first ionic liquid solvent to dissolve more lithium salt than it can without such a solubility-promoting agent. Typically, a lithium salt can be dissolved in an electrolyte solvent (particularly ionic liquid solvent) up to a concentration less than 2 M, more typically less than 1.5 M, and most typically between 0.3 M and 1.0 M. Quite unexpectedly, the addition of a judicially selected solubility-promoting agent can significantly increase the solubility limit to typically greater than 5 M and, in several cases, greater than 10 M.

The solubility-promoting agent to be used in an ionic liquid-based electrolyte may advantageously contain an ether-based organic solvent, such as 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), Poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, a combination thereof, or a combination with a carbonate-based solvent. It is of interest to point out that an ether-based organic solvent itself is typically not capable of dissolving more than 2 M of lithium salt. Since the ionic liquid solvent is also incapable of dissolving more than 1.5 M of a lithium salt, a mixture solvent (with an IL-to-organic ratio of from 3:1 to 10:1) is expected to be incapable of dissolving >3M of lithium salt. (Actually, an electrochemist or battery designer would not be motivated to add more than 1.5M of lithium salt into an ionic liquid since ionic liquids are known to be already highly viscous).

The amount of the solubility-promoting agent is an important consideration. Typically, a more effective solubility-promoting agent itself tends to be more volatile or flammable. For instance, ether-based organic solvents listed above are very effective solubility promoters for ionic liquids, but they are also highly flammable. Fortunately, after an intensive search and research, we have discovered that it is not necessary to add much ether in an ionic liquid to make the mixture solvent capable of dissolving large proportions of a lithium salt. Typically, an IL-to-organic solvent ratio of from 11:1 to 5:1 is sufficient. In some exceptional cases (e.g. PEGDME), the required IL-to-organic ratio is approximately 4:1-3:1. Even in the latter ratio (3:1 or 25% PEGDME in 75% IL, imidazolium bis(trifluoromethanesulphonyl)imide), the resulting mixture electrolyte is not flammable and, thus, is safe. These unexpected observations are scientifically and technologically very significant.

The solubility-promoting agent in the ionic liquid-based electrolyte may also be selected from carbonate-based organic material, such as ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene or methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), or a combination thereof.

Thus, in one embodiment of the present invention, the ratio of the solubility-promoting agent amount to the first ionic liquid solvent amount is less than 1/3, preferably less than 1/5, more preferably less than 1/9, and most preferably less than 1/10. One could choose to add an initially excess amount of the solubility-promoting agent amount to the mixture to achieve an ultra-high lithium salt concentration. This is then followed by partial or complete removal of the volatile or flammable ingredients in the electrolyte after the electrolyte is combined with an anode or cathode. The solubility-promoting agent may be selected from an organic liquid or a second ionic liquid, provided it can significantly increase the solubility limit of a lithium salt in the first ionic liquid solvent.

The anode active material may contain, as an example, lithium metal foil or a high-capacity Si, Sn, or SnO₂ capable of storing a great amount of lithium. For Li—S cells, the cathode active material may contain pure sulfur (if the anode active material contains lithium), lithium polysulfide, or any sulfur containing compound, molecule, or polymer. If the cathode active material includes lithium-containing species (e.g. lithium polysulfide) when the cell is made, the anode active material can be any material capable of storing a large amount of lithium (e.g. Si, Ge, Sn, SnO₂, etc). For other lithium secondary cells, the cathode active materials can include a transition metal dichalcogenide (e.g., TiS₂, TaS₂, and MoS₂), a transition metal trichalcogenide (e.g., NbSe₃), a transition metal oxide (e.g., MnO₂, CoO₂, an iron oxide, a vanadium oxide, etc), or a combination thereof. The vanadium oxide may be selected from the group consisting of VO₂, Li_(x)VO₂, V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.

The rechargeable lithium metal or lithium-ion cell featuring an ionic liquid solvent-based electrolyte containing a high lithium salt concentration may contain a cathode active material selected from a layered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

Typically, the cathode active materials are not electrically conducting. Hence, in one embodiment, the cathode active material may be mixed with a conductive filler such as carbon black (CB), acetylene black (AB), graphite particles, expanded graphite particles, activated carbon, meso-porous carbon, meso-carbon micro bead (MCMB), carbon nano-tube (CNT), carbon nano-fiber (CNF), graphene sheet (also referred to as nano graphene platelet, NGP), carbon fiber, or a combination thereof. These carbon/graphite/graphene materials may be made into a form of fabric, mat, or paper for supporting the cathode active material.

In a preferred embodiment, the nano-scaled filaments (e.g. CNTs, CNFs, and/or NGPs) are formed into a porous nano-structure that contains massive surfaces to support either the anode active material (e.g. Li or Si coating) or the cathode active material (e.g. sulfur, lithium polysulfide, vanadium oxide, TiS₂, etc). The porous nano-structure should have pores having a pore size preferably from 2 nm to 1 μm prior to being impregnated with sulfur or lithium polysulfide. The pore size is preferably in the range of 2 nm-50 nm, further preferably 2 nm-10 nm, after the pores are impregnated with sulfur or lithium polysulfide. These pores are properly sized to accommodate the electrolyte at the cathode side and to retain the cathode active material in the pores during repeated charges/discharges. The same type of nano-structure may be implemented at the anode side to support the anode active material.

In another preferred embodiment, the cathode active material consists of (a) exfoliated graphite worms that are interconnected to form a porous, conductive graphite flake network comprising pores having a size smaller than 100 nm; and (b) nano-scaled powder or coating of sulfur, sulfur compound, or lithium polysulfide disposed in the pores or coated on a graphite flake surface wherein the powder or coating is in contact with the electrolyte and has a dimension less than 100 nm. Preferably, the exfoliated graphite worm amount is in the range of 1% to 90% by weight and the amount of powder or coating is in the range of 99% to 10% by weight based on the total weight of exfoliated graphite worms and sulfur, sulfur compound, or lithium polysulfide combined which is measured or calculated when the cell is in a fully charged state. Preferably, the amount of the powder or coating of sulfur, sulfur compound, or lithium polysulfide is in the range of 70% to 95% by weight. Most preferably, the amount of the powder or coating of sulfur, sulfur compound, or lithium polysulfide is no less than 80% by weight.

The electrons coming from or going out through the external load or circuit must go through the conductive additives (in a conventional sulfur cathode) or a conductive framework (e.g. exfoliated graphite meso-porous structure or nano-structure of conductive nano-filaments) to reach the cathode active material. Since the cathode active material (e.g. sulfur, lithium polysulfide, vanadium oxide, etc) is a poor electronic conductor, the active material particle or coating must be as thin as possible to reduce the required electron travel distance.

Conventional Li—S cells typically have been limited to less than 70% by weight of sulfur in a composite cathode composed of sulfur and the conductive additive/support. Even when the sulfur content in the prior art composite cathode reaches or exceeds 70% by weight, the specific capacity of the composite cathode is typically significantly lower than what is expected based on theoretical predictions. For instance, the theoretical specific capacity of sulfur is 1,675 mAh/g. A composite cathode composed of 70% sulfur (S) and 30% carbon black (CB), without any binder, should be capable of storing up to 1,675×70%=1,172 mAh/g. Unfortunately, the actually observed specific capacity is typically less than 75% (often less than 50%) of what can be achieved. In other words, the active material utilization rate is typically less than 75% (or even <50%). This has been a major issue in the art of Li—S cells and there has been no solution to this problem. Most surprisingly, the implementation of exfoliated graphite worms as a conductive supporting material for sulfur or lithium polysulfide, coupled with an ionic liquid electrolyte at the cathode, has made it possible to achieve an active material utilization rate of typically >>80%, more often greater than 90%, and, in many cases, close to 99%.

In the presently invented lithium-sulfur cell, the pores of the porous sulfur/exfoliated graphite mixture or composite preferably have a size from 2 nm to 10 nm to accommodate electrolyte therein after the nano-scaled powder or coating of sulfur, sulfur compound, or lithium polysulfide is disposed in the pores or coated on the graphite flake surface. These pore sizes in the sulfur/exfoliated graphite mixture or composite are surprisingly capable of further suppressing, reducing, or eliminating the shuttle effect. Not wishing to be bound by the theory, but we feel that this is likely due to the unexpected capability of exfoliated graphite flake surfaces spaced 2-10 nm apart to retain lithium polysulfides in the minute pockets (pores) during the charge and discharge cycles. This ability of graphitic surfaces to prevent out-migration of lithium polysulfide is another big surprise to us.

The exfoliated graphite worms can be obtained from the intercalation and exfoliation of a laminar graphite material, explained as follows: Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite crystal or crystallite), or a whole range of intermediate structures that are characterized by having various proportions and sizes of graphite crystallites and defects dispersed in an amorphous carbon matrix. Typically, a graphite crystallite is composed of a number of graphene sheets or basal planes (also referred to as a-b planes) that are bonded together through van der Waals forces in the c-axis direction, the direction perpendicular to the basal plane. Each graphene plane is a plane of carbon atoms arranged in a hexagonal structure. These graphite crystallites are typically micron- or nanometer-sized in the a- or b-direction (these are called La dimension). The c-directional dimension (or thickness) is commonly referred to as Lc. The interplanar spacing of a perfect graphite is known to be approximately 0.335 nm (3.35 Å). The graphite crystallites are dispersed in or connected by crystal defects or an amorphous phase in a laminar graphite particle, which can be a graphite particle (natural graphite or synthetic, such as pyrolytic graphite), graphite spherule (spheroidal graphite or micro graphite ball), carbon/graphite fiber segment, carbon/graphite whisker, carbon/graphite nano-fiber (CNF or GNF), and meso-phase micro-bead (MCMB).

The conventional process for producing exfoliated graphite worms typically begins with subjecting a graphitic material to a chemical treatment (intercalation and/or oxidation using a strong acid and/or oxidizing agent) to form a graphite intercalation compound (GIC) or graphite oxide (GO). This is most often accomplished by immersing natural graphite powder in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium chlorate). The resulting GIC is actually some type of graphite oxide (GO) particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water. There are different processing routes that can be followed after this rinsing step to form different types of graphite or graphene products.

For instance, a first route involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of 30-800 to form “graphite worms”, which are each a collection of exfoliated, but largely un-separated or still interconnected graphite flakes. Representative SEM images of graphite worms are presented in FIG. 1(A)-1(B).

As a second route, one may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite flakes,” which are isolated and separated graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition). Alternatively, exfoliated graphite worms may be the re-compressed (e.g. roll-pressed) to form flexible graphite sheet or flexible graphite foil that is essentially a solid film not permeable to battery electrolyte. Such an electrolyte-impermeable film can be a good battery current collector (e.g. to replace aluminum foil), but it does not have a sufficient amount of specific surface area to support sulfur.

Alternatively, as a third route, the exfoliated graphite worms may be subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and/or multi-layer graphene sheets (collectively called nano graphene platelets or NGPs), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm.

The graphite oxide suspension (after a sufficiently high degree of oxidation) may be subjected to ultrasonication for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation bas been increased from 0.335 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets having an oxygen content of typically 20-50% by weight. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.01%-10% by weight, more typically 0.01%-5% by weight, and most typically 0.01%-2% by weight.

In general, NGPs include single-layer and multi-layer graphene or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% weight. Pristine graphene has essentially 0% oxygen. Graphene oxide (including RGO) can have 0.01%-50% by weight of oxygen.

As indicated earlier, dried GIC or GO powder may be exposed a thermal shock (at a high temperature, typically 800-1,050° C.) for a short period of time (typically 30-120 seconds), allowing the constituent graphite flakes to freely expand. The resulting graphite worms typically have an expanded volume that is 30 to 800 times higher than the original graphite volume, depending upon the degree of oxidation or intercalation.

Typically, an oxygen content between 46-50% by weight based on the total GO weight is an indication of practically complete oxidation of graphite, which is also reflected by the complete disappearance of the X-ray diffraction curve peak originally located at 2θ=approximately 26 degrees for un-intercalated or un-oxidized natural graphite. This diffraction peak at 2θ=approximately 26 degrees corresponds to the d_(o02) spacing between two (002) graphene planes.

Acids, such as sulfuric acid, are not the only type of intercalating agent (intercalant) that penetrate into spaces between graphene planes. Many other types of intercalating agents, such as alkali metals (Li, K, Na, Cs, and their alloys or eutectics), can be used to intercalate graphite to stage 1, stage 2, stage 3, etc. Stage n implies one intercalant layer for every n graphene planes. For instance, a stage-1 potassium-intercalated GIC means there is one layer of K for every graphene plane; or, one can find one layer of K atoms inserted between two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence, where G is a graphene plane and K is a potassium atom plane. A stage-2 GIC will have a sequence of GG/K/GG/K/GG/K/GG . . . and a stage-3 GIC will have a sequence of GGG/K/GGG/K/GGG . . . , etc.

A graphite worm is characterized as having a network of largely interconnected exfoliated graphite flaks with pores between flakes. The flakes have a typical length or width dimension of 0.5-100 μm (more typically 1-20 μm), depending upon the types of starting graphitic materials used and these lateral dimensions (length or width) are relatively independent of the GIC stage number (or oxygen content in GO), the exfoliation temperature, and the exfoliation environment. However, these factors have major impact on the volume expansion ratio (exfoliated graphite worm volume vs. starting graphite particle volume), flake thickness range, and pore size range of exfoliated graphite worms.

For instance, Stage-1 GIC or fully oxidized graphite (GO with 40-50% oxygen content), upon un-constrained exfoliation at 1,000° C. for one minute, exhibit a typical volume expansion ratio of approximately 450-800%, flake thickness range of 0.34 to 3 nm, and pore size range of 50 nm to 20 μM. By contrast, Stage-5 GIC or GO with 20-25% oxygen content, upon un-constrained exfoliation at 1,000° C. for one minute, exhibit a volume expansion ratio of approximately 80-180%, flake thickness range of 1.7 to 200 nm, and pore size range of 30 nm to 2 μm.

Stage-1 GIC is the most desirable since it leads to highly exfoliated graphite worms featuring thin graphite flakes with very high specific surface areas (typically >500 m²/g, often >700 m²/g, and even >1,000 m²/g in several cases). Higher surface areas make it possible to deposit thinner sulfur or lithium polysulfide coating given the same sulfur or lithium polysulfide volume. Consequently, there is significantly reduced proportion of thicker coating of sulfur or lithium polysulfide attached to the exfoliated graphite flake surfaces. This could allow most of the sulfur to be accessible to the lithium ions during the cell discharge.

The flakes in an exfoliated graphite worm remain substantially interconnected (physically in contact with each other or bonded to each other), forming a network of electron-conducting paths. Hence, the electrical conductivity of the graphite worms is relatively high (10-10,000 S/cm), which can be orders of magnitude higher than that of carbon black, activated carbon, polymeric carbon, amorphous carbon, hard carbon, soft carbon, and meso-phase pitch, etc.

The soft and fluffy worms, upon impregnation or coating with sulfur, have exhibited an unexpected improvement in mechanical strength (e.g. compression strength or bending strength) by up to 2-3 orders of magnitude. The impregnated graphite worms may be re-compressed to increase their physical density and structural integrity, if deemed necessary. Graphite worm-sulfur composites have a density typically in the range of 0.02 g/cm³ to 1.0 g/cm³, depending upon the degree of exfoliation and the condition of re-compression.

When the cathode is made, the cathode active material (sulfur, lithium polysulfide, vanadium oxide, titanium disulfide, etc) is embedded in the nano-scaled pores constituted by the exfoliated graphite flakes. Preferably, the cathode active material is grinded into nanometer scale (preferably <10 nm and more preferably <5 nm). Alternatively, the cathode active material may be in a thin-film coating form deposited on surfaces of the graphite flakes obtained by melt impregnation, solution deposition, electro-deposition, chemical vapor deposition (CVD), physical vapor deposition, sputtering, laser ablation, etc. This coating is then brought in contact with electrolyte before, during, or after the cathode is made, or even after the cell is produced.

The present design of a meso-porous graphite worm cathode with meso-scaled pores in a Li—S cell was mainly motivated by the notion that a significant drawback with cells containing cathodes comprising elemental sulfur, organosulfur and carbon-sulfur materials is related to the dissolution and excessive out-diffusion of soluble sulfides, polysulfides, organo-sulfides, carbon-sulfides and/or carbon-polysulfides (anionic reduction products) from the cathode into the rest (anode, in particular) of the cell. This process leads to several problems: high self-discharge rates, loss of cathode capacity, corrosion of current collectors and electrical leads leading to loss of electrical contact to active cell components, fouling of the anode surface giving rise to malfunction of the anode, and clogging of the pores in the cell membrane separator which leads to loss of ion transport and large increases in internal resistance in the cell.

At the anode side, when lithium metal is used as the sole anode active material in a Li metal cell, there is concern about the formation of lithium dendrites, which could lead to internal shorting and thermal runaway. Herein, we have used two approaches, separately or in combination, to addressing this dendrite formation issue: one involving the use of a high-concentration electrolyte and the other the use of a nano-structure composed of conductive nano-filaments. For the latter, multiple conductive nano-filaments are processed to form an integrated aggregate structure, preferably in the form of a closely packed web, mat, or paper, characterized in that these filaments are intersected, overlapped, or somehow bonded (e.g., using a binder material) to one another to form a network of electron-conducting paths. The integrated structure has substantially interconnected pores to accommodate electrolyte. The nano-filament may be selected from, as examples, a carbon nano fiber (CNF), graphite nano fiber (GNF), carbon nano-tube (CNT), metal nano wire (MNW), conductive nano-fibers obtained by electro-spinning, conductive electro-spun composite nano-fibers, nano-scaled graphene platelet (NGP), or a combination thereof. The nano-filaments may be bonded by a binder material selected from a polymer, coal tar pitch, petroleum pitch, meso-phase pitch, coke, or a derivative thereof.

Surprisingly and significantly, the nano-structure provides an environment that is conducive to uniform deposition of lithium atoms, to the extent that no geometrically sharp structures or dendrites were found in the anode after a large number of cycles. Not wishing to be bound by any theory, but the applicants envision that the 3-D network of highly conductive nano-filaments provide a substantially uniform attraction of lithium ions back onto the filament surfaces during re-charging. Furthermore, due to the nanometer sizes of the filaments, there is a large amount of surface area per unit volume or per unit weight of the nano-filaments. This ultra-high specific surface area offers the lithium ions an opportunity to uniformly deposit a lithium metal coating on filament surfaces at a high rate, enabling high re-charge rates for a lithium metal secondary battery.

The presently invented high-concentration electrolyte system and optional meso-porous exfoliated graphite-sulfur may be incorporated in several broad classes of rechargeable lithium cells. In the following examples, sulfur or lithium polysulfide is used as a cathode active material for illustration purposes:

-   -   (A) Lithium metal-sulfur with a conventional anode         configuration: The cell contains an optional cathode current         collector, a cathode (containing a composite of sulfur or         lithium polysulfide and a conductive additive or a conductive         supporting framework, such as a meso-porous exfoliated graphite         or a nano-structure of conductive nano-filaments), a         separator/electrolyte (featuring the gradient electrolyte         system), and an anode current collector. Potential dendrite         formation may be overcome by using the high-concentration         electrolyte at the anode.     -   (B) Lithium metal-sulfur cell with a nano-structured anode         configuration: The cell contains an optional cathode current         collector, a cathode (containing a composite of sulfur or         lithium polysulfide and a conductive additive or a conductive         supporting framework, such as a meso-porous exfoliated graphite         or a nano-structure of conductive nano-filaments), a         separator/electrolyte (featuring the gradient electrolyte         system), an optional anode current collector, and a         nano-structure to accommodate lithium metal that is deposited         back to the anode during a charge or re-charge operation. This         nano-structure (web, mat, or paper) of nano-filaments provide a         uniform electric field enabling uniform Li metal deposition,         reducing the propensity to form dendrites. This configuration,         coupled with the high-concentration electrolyte at the anode,         provides a dendrite-free cell for a long and safe cycling         behavior.     -   (C) Lithium ion-sulfur cell with a conventional anode: For         instance, the cell contains an anode composed of anode active         graphite particles bonded by a binder, such as polyvinylidene         fluoride (PVDF) or styrene-butadiene rubber (SBR). The cell also         contains a cathode current collector, a cathode (containing a         composite of sulfur or lithium polysulfide and a conductive         additive or a conductive supporting framework, such as a         meso-porous exfoliated graphite or a nano-structure of         conductive nano-filaments), a separator/electrolyte (featuring         the gradient electrolyte system), and an anode current         collector; and     -   (D) Lithium ion-sulfur cell with a nano-structured anode: For         instance, the cell contains a web of nano-fibers coated with Si         coating or bonded with Si nano particles. The cell also contains         an optional cathode current collector, a cathode (containing a         composite of sulfur or lithium polysulfide and a conductive         additive or a conductive supporting framework, such as a         meso-porous exfoliated graphite or a nano-structure of         conductive nano-filaments), a separator/electrolyte (featuring         the gradient electrolyte system), and an anode current         collector. This configuration provides an ultra-high capacity,         high energy density, and a safe and long cycle life.         This sulfur or lithium polysulfide in (A)-(D) can be replaced         with any other type of cathode active materials, such as a         transition metal dichalcogenide (e.g., TiS₂), transition metal         trichalcogenide (e.g., NbSe₃), transition metal oxide (e.g.,         MnO₂, a vanadium oxide, etc), a layered compound LiMO₂, spinel         compound LiM₂O₄, olivine compound LiMPO₄, silicate compound         Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or         a combination thereof, wherein M is a transition metal or a         mixture of multiple transition metals

In the lithium-ion sulfur cell (e.g. as described in (C) and (D) above), the anode active material can be selected from a wide range of high-capacity materials, including (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe) and cadmium (Cd), and lithiated versions thereof; (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiated versions thereof, wherein said alloys or compounds are stoichiometric or non-stoichiometric; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, and lithiated versions thereof; (d) salts and hydroxides of Sn and lithiated versions thereof; (e) carbon or graphite materials and prelithiated versions thereof; and combinations thereof. Non-lithiated versions may be used if the cathode side contains lithium polysulfides or other lithium sources when the cell is made.

A possible lithium metal cell may be comprised of an anode current collector, a electrolyte phase (optionally but preferably supported by a porous separator, such as a porous polyethylene-polypropylene co-polymer film), a meso-porous exfoliated graphite worm-sulfur cathode of the instant invention (containing a cathode active material), and an optional cathode collector. This cathode current collector is optional because the presently invented meso-porous exfoliated graphite structure, if properly designed, can act as a current collector or as an extension of a current collector.

To achieve high capacity in a battery, it is desirable to have either a higher quantity or loading of the cathode active material or, preferably, a higher-capacity cathode active material in the cathode layer. Lithium and sulfur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, because they provide nearly the highest energy density possible on a weight or volume basis of any of the known combinations of active materials (other than the Li-air cell). To obtain high energy densities, the lithium can be present as the pure metal, in an alloy (in a lithium-metal cell), or in an intercalated form (in a lithium-ion cell), and the sulfur can be present as elemental sulfur or as a component in an organic or inorganic material with a high sulfur content.

With sulfur-based compounds, which have much higher specific capacities than the transition metal oxides of lithium-ion cells, it is difficult to obtain efficient electrochemical utilization of the sulfur-based compounds at high volumetric densities because the sulfur-based compounds are highly insulating and are generally not micro-porous. For example, U.S. Pat. No. 5,532,077 to Chu describes the problems of overcoming the insulating character of elemental sulfur in composite cathodes and the use of a large volume fraction of an electronically conductive material (carbon black) and of an ionically conductive material (e.g., polyethylene oxide or PEO) in the composite electrode to try to overcome these problems. Typically, Chu had to use nearly 50% or more of non-active materials (e.g., carbon black, binder, PEO, etc), effectively limiting the relative amount of active sulfur. Furthermore, presumably one could choose to use carbon paper (instead of or in addition to carbon black) as a host for the cathode active material. However, this conventional carbon fiber paper does not allow a sufficient amount of cathode active material to be coated on the large-diameter carbon fiber surface yet still maintaining a low coating thickness, which is required of a reduced lithium diffusion path length for improved charge/discharge rates and reduced resistance. In other words, in order to have a reasonable proportion of an electrode active material coated on a large-diameter fiber, the coating thickness has to be proportionally higher. A thicker coating would mean a longer diffusion path for lithium to come in and out, thereby slowing down the battery charge/discharge rates. The instant application solved these challenging problems by using an integrated 3-D meso-porous graphite worm structure consisting of nano-thickness exfoliated graphite flakes having massive conductive surfaces to host the cathode active material (sulfur, sulfur-containing compound, or lithium polysulfide).

As opposed to carbon paper (often used as a host for elemental sulfur, conductive additives, ion conductors, and electrolyte) that was composed of micron-scaled carbon fibers (typically having a diameter of >12 μm), the instant application makes use of graphite worms of nano-thickness flakes with a thickness less than 200 nm, preferably and more typically less than 100 nm, even more preferably and more typically less than 10 nm, and most preferably and more typically less than 3 nm. The exfoliated graphite worms have been ignored or overlooked by the workers in the art of designing electrodes likely due to the notion that these worms are perceived as too weak to be handled in an electrode-making process and too weak to support any sulfur-containing electrode active material. Indeed, graphite worms are extremely weak. However, impregnation of coating of graphite worms with sulfur or sulfur compounds significantly enhances the mechanical strength of graphite worms, to the extent that the resulting composite materials can be readily formed into a cathode using a conventional battery electrode-making machine (coater). Further, there has been no teaching that exfoliated graphite worms could be used to confine lithium polysulfide and preventing lithium polysulfide from migrating out of the cathode and entering the anode. This was not trivial or obvious to one of ordinary skills in the art.

The interconnected network of exfoliated graphite worms forms a continuous path for electrons, resulting in significantly reduced internal energy loss or internal heating for either the anode or the cathode (or both). This network is electronically connected to a current collector and, hence, all graphite flakes that constitute graphite worms are essentially connected to the current collector. In the instant invention, the lithium sulfide coating is deposited on flake surfaces and, even if the coating were to fracture into separate segments, individual segments would still remain in physical contact with the underlying flakes, which is essentially part of the current collector. The electrons transported to the cathode can be distributed to all cathode active coatings. In the case of lithium sulfide particles dispersed/dissolved in an electrolyte inside meso pores of the cathode structure, the particles are necessarily nano-scaled (the salt-electrolyte solution pool also nano-scaled) and, hence, are conducive to fast cathode reaction during the charging operation.

The lithium metal cell of the instant application can have a nano-structured anode or a more conventional anode structure, although such a conventional structure is not preferred. In a more conventional anode structure, acetylene black, carbon black, or ultra-fine graphite particles may be used as a conductive additive. The binder may be chosen from polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), or styrene-butadiene rubber (SBR), for example. Conductive materials such as electronically conductive polymers, meso-phase pitch, coal tar pitch, and petroleum pitch may also be used as a binder. Preferable mixing ratio of these ingredients may be 80 to 95% by weight for the anode active material (natural or artificial graphite particles, MCMBs, coke-based anode particles, carbon-coated Si nano particles, etc), 3 to 20% by weight for the conductive additive, and 2 to 7% by weight for the binder. The anode current collector may be selected from copper foil or stainless steel foil. The cathode current collector may be an aluminum foil or a nickel foil. There is no particularly significant restriction on the type of current collector, provided the material is a good electrical conductor and relatively corrosion resistant. The separator may be selected from a polymeric nonwoven fabric, porous polyethylene film, porous polypropylene film, or porous PTFE film.

The most important property of a filament herein used to support an electrode active material (e.g. Li or Si at the anode) is a high electrical conductivity to enable facile transport of electrons with minimal resistance. A low conductivity implies a high resistance and high energy loss, which is undesirable. The filament should also be chemically and thermo-mechanically compatible with the intended active material (i.e., lithium at the anode) to ensure a good contact between the filament and the coating upon repeated charging/discharging and heating/cooling cycles. Several techniques can be employed to fabricate a conductive aggregate of filaments (a web or mat), which is a monolithic body having desired interconnected pores. In one preferred embodiment of the present invention, the porous web can be made by using a slurry molding or a filament/binder spraying technique. These methods can be carried out in the following ways:

EXAMPLES

In the examples discussed below, unless otherwise noted, raw materials such as silicon, germanium, bismuth, antimony, zinc, iron, nickel, titanium, cobalt, and tin were obtained from either Alfa Aesar of Ward Hill, Mass., Aldrich Chemical Company of Milwaukee, Wis. or Alcan Metal Powders of Berkeley, Calif. X-ray diffraction patterns were collected using a diffractometer equipped with a copper target x-ray tube and a diffracted beam monochromator. The presence or absence of characteristic patterns of peaks was observed for each of the alloy samples studied. For example, a phase was considered to be amorphous when the X-ray diffraction pattern was absent or lacked sharp, well-defined peaks. In several cases, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the structure and morphology of the hybrid material samples.

A nano-structured cathode, comprising exfoliated graphite worm-sulfur (or polysulfide), was bonded onto an aluminum foil (a current collector). After solvent removal, web-aluminum foil configuration was hot-pressed to obtain a cathode or, alternatively, a complete cell was fabricated by laminating an anode current collector (Cu foil), an anode layer (e.g., a piece of Li foil, a nano-structured web with Si coating, or graphite particles bonded by PVDF), an electrolyte-separator layer, a meso-porous cathode, and a cathode current collector (e.g., stainless steel foil or aluminum foil) all at the same time. In some cases, an NGP-containing resin was used as a binder, for instance, between a cathode layer and a cathode current collector. Filaments may also be bonded by an intrinsically conductive polymer as a binder to form a web. For instance, polyaniline-maleic acid-dodecyl hydrogensulfate salt may be synthesized directly via emulsion polymerization pathway using benzoyl peroxide oxidant, sodium dodecyl sulfate surfactant, and maleic acid as dopants. Dry polyaniline-based powder may be dissolved in DMF up to 2% w/v to form a solution.

The conventional cathode of a Li—S cell was prepared in the following way. As an example, 60% by weight of lithium sulfide powder, 3.5% by weight of acetylene black, 33.5% by weight of graphite, and 3% by weight of ethylene-propylene-diene monomer powder were mixed together with toluene to obtain a mixture. The mixture was then coated on an aluminum foil (30 μm) serving as a current collector. The resulting two-layer aluminum foil-active material configuration was then hot-pressed to obtain a positive electrode. In the preparation of a cylindrical cell, a positive electrode, a separator composed of a porous polyethylene film, and a negative electrode was stacked in this order. The stacked body was spirally wound with a separator layer being disposed at the outermost side to obtain an electrode assembly. For Li-ion cells were similarly made wherein, for instance, the cathode is prepared by mixing 90% by weight of a selected cathode active material with 5% conductive additive (e.g. carbon black), and 5% binder (e.g. PVDF).

The following examples are presented primarily for the purpose of illustrating the best mode practice of the present invention, not to be construed as limiting the scope of the present invention.

Example 1 Some Examples of Electrolytes Used

A wide range of lithium salts can be used as the lithium salt dissolved in an ionic liquid solvent (alone or as a mixture with another ionic liquid or a solubility-promoting liquid). The following are good choices for lithium salts that tend to be dissolved well in selected ionic liquid solvents: lithium borofluoride (LiBF₄), lithium trifluoro-metasulfonate (LiCF₃SO₃), lithium bis-trifluoromethyl sulfonylimide (LiN(CF₃SO₂)₂ or LITFSI), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), and lithium bisperfluoroethy-sulfonylimide (LiBETI). A good electrolyte additive for helping to stabilize Li metal is LiNO₃.

Preferred ionic liquid solvents may be selected from a room temperature ionic liquid (RTIL) having a cation selected from tetraalkylammonium, di-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, or dialkylpiperidinium. The counter anion is preferably selected from BF₄ ⁻, B(CN)₄ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, or N(SO₂F)₂ ⁻. Particularly useful ionic liquid-based electrolytes include: lithium bis(trifluoro methanesulfonyl)imide in a N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethane sulfonyl)imide (LiTFSI in BEPyTFSI), N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl)imide (PP₁₃TFSI) containing LiTFSI, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEMETFSI) containing LiTFSI.

Preferred solubility-promoting liquids include: ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), propylene carbonate (PC), acetonitrile (AN), vinylene carbonate (VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), Poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, and sulfolane.

Example 2 Exfoliated Graphite Worms from Natural Graphite Using Hummers Method

Graphite intercalation compound (GIC) was prepared by intercalation and oxidation of natural graphite flakes (original size of 200 mesh, from Huadong Graphite Co., Pingdu, China, milled to approximately 15 μm) with sulfuric acid, sodium nitrate, and potassium permanganate according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite, we used a mixture of 22 ml of concentrated sulfuric acid, 2.8 grams of potassium permanganate, and 0.5 grams of sodium nitrate. The graphite flakes were immersed in the mixture solution and the reaction time was approximately three hours at 30° C. It is important to caution that potassium permanganate should be gradually added to sulfuric acid in a well-controlled manner to avoid overheat and other safety issues. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The sample was then washed repeatedly with deionized water until the pH of the filtrate was approximately 5. The slurry was spray-dried and stored in a vacuum oven at 60° C. for 24 hours. The resulting GIC was exposed to a temperature of 1,050° C. for 35 seconds in a quartz tube filled with nitrogen gas to obtain worms of exfoliated graphite flakes.

Example 3 Conductive Web of Filaments from Electro-Spun PAA Fibrils for Anode

Poly (amic acid) (PAA) precursors for spinning were prepared by copolymerizing of pyromellitic dianhydride (Aldrich) and 4,4′-oxydianiline (Aldrich) in a mixed solvent of tetrahydrofurane/methanol (THF/MeOH, 8/2 by weight). The PAA solution was spun into fiber web using an electrostatic spinning apparatus. The apparatus consisted of a 15 kV d.c. power supply equipped with the positively charged capillary from which the polymer solution was extruded, and a negatively charged drum for collecting the fibers. Solvent removal and imidization from PAA were performed concurrently by stepwise heat treatments under air flow at 40° C. for 12 h, 100° C. for 1 h, 250° C. for 2 h, and 350° C. for 1 h. The thermally cured polyimide (PI) web samples were carbonized at 1,000° C. to obtain a sample with an average fibril diameter of 67 nm. Such a web can be used to accommodate sulfur (or lithium polysulfide), vanadium oxide, titanium disulfide, etc., for the cathode and/or as a conductive substrate for an anode active material.

Example 4 Preparation of NGP-Based Webs (Webs of NGPs and NGPs+CNFs) for the Anode or Cathode (as a Conductive Nanostructured Support)

The starting natural graphite flakes (original size of 200 mesh, from Huadong Graphite Co., Pingdu, China) was milled to approximately 15 μm. The intercalation and oxidation chemicals used in the present study, including fuming nitric acid (>90%), sulfuric acid (95-98%), potassium chlorate (98%), and hydrochloric acid (37%), were purchased from Sigma-Aldrich and used as received.

A reaction flask containing a magnetic stir bar was charged with sulfuric acid (360 mL) and nitric acid (180 mL) and cooled by immersion in an ice bath. The acid mixture was stirred and allowed to cool for 15 min, and graphite particles (20 g) were added under vigorous stirring to avoid agglomeration. After the graphite particles were well dispersed, potassium chlorate (110 g) was added slowly over 15 min to avoid sudden increases in temperature. The reaction flask was loosely capped to allow evolution of gas from the reaction mixture, which was stirred for 48 hours at room temperature. On completion of the reaction, the mixture was poured into 8 L of deionized water and filtered. The slurry was spray-dried to recover an expandable graphite sample. The dried, expandable graphite sample was quickly placed in a tube furnace preheated to 1,000° C. and allowed to stay inside a quartz tube for approximately 40 seconds to obtain exfoliated graphite worms. The worms were dispersed in water to form a suspension, which was ultrasonicated with a power of 60 watts for 15 minutes to obtain separated NGPs.

Approximately half of the NGP-containing suspension was filtered and dried to obtain several paper-like mats. Vapor grown CNFs were then added to the remaining half to form a suspension containing both NGPs and CNFs (20%), which was dried and made into several paper-like mats. Approximately 5% phenolic resin binder was used to help consolidate the web structures in both samples. Such a web can be as a conductive substrate for an anode active material.

Example 5 Physical Vapor Deposition (PVD) of Sulfur on Meso-Porous Graphite Worm Conductive Structures for Li—S Cathodes

In a typical procedure, a meso-porous graphite worm structure or a nano-filament web is sealed in a glass tube with the solid sulfur positioned at one end of the glass tube and the web near another end at a temperature of 40-75° C. The sulfur vapor exposure time was typically from several minutes to several hours for a sulfur coating of several nanometers to several microns in thickness. A sulfur coating thickness lower than 100 nm is preferred, but more preferred is a thickness lower than 20 nm, and most preferred is a thickness lower than 10 nm (or even 5 nm). Several lithium metal cells with or without a nano-structured anode were fabricated, wherein a lithium metal foil was used as a Li ion source.

Example 6 Preparation of Graphene-Enabled Li—V₃O₈ Nano-Sheets from V₂O₅ and LiOH

All chemicals used in this study were analytical grade and were used as received without further purification. V₂O₅ (99.6%, Alfa Aesar) and LiOH (99+%, Sigma-Aldrich) were used to prepare the precursor solution. Graphene oxide (GO, 1% w/v obtained in Example 2 above) was used as a structure modifier. First, V₂O₅ and LiOH in a stoichiometric V/Li ratio of 1:3 were dissolved in actively stirred de-ionized water at 50° C. until an aqueous solution of Li_(x)V₃O₈ was formed. Then, GO suspension was added while stirring, and the resulting suspension was atomized and dried in an oven at 160° C. to produce the spherical composite particulates of GO/Li_(x)V₃O₈ nano-sheets and the sample was designated NLVO-1. Corresponding Li_(x)V₃O₈ materials were obtained under comparable processing conditions, but without graphene oxide sheets. The sample was designated as LVO-2.

An additional set of graphene-enabled Li_(x)V₃O₈ nano-sheet composite particulates was produced from V₂O₅ and LiOH under comparable conditions, but was dried under different atomization temperatures, pressures, and gas flow rates to achieve four samples of composite particulates with four different Li_(x)V₃O₈ nano-sheet average thicknesses (4.6 nm, 8.5 nm, 14 nm, and 35 nm). A sample of Li_(x)V₃O₈ sheets/rods with an average thickness/diameter of 76 nm was also obtained without the presence of graphene oxide sheets (but, with the presence of carbon black particles) under the same processing conditions for the graphene-enhanced particulates with a nano-sheet average thickness of 35 nm. It seems that carbon black is not as good a nucleating agent as graphene for the formation of Li_(x)V₃O₈ nano-sheet crystals. The specific capacities and other electrochemical properties of these cathode materials in Li metal cells using lithium foil as a counter electrode and in Li-ion cells using a graphite anode were investigated.

Example 7 Hydrothermal Synthesis of Graphene-Enabled V₃O₇H₂O Nano-Belts from V₂O₅ and Graphene Oxide

In a typical procedure, 0.015 g of V₂O₅ was added into 9 ml of distilled water. A GO-water suspension (V₂O₅/GO ratio of 98/2) was poured into the V₂O₅ suspension. The resulting mixture was transferred to a 35 ml Teflon-sealed autoclave and stored at 180-200° C. for 24-36 h (different batches), then was air-cooled to room temperature. GO was used as a heterogeneous nucleation agent to promote fast nucleation of larger numbers of nuclei for reduced crystallite sizes (promote nucleation against growth of crystals). The products were washed several times with distilled water, and finally dried at 60° C. in an oven.

A second batch was obtained by spray-drying at 200° C. and heat-treated at 400° C. for 2 hours to obtain particulates of GO/V₃O₇H₂O composite with graphene oxide sheets embracing around these particulates. For comparison purposes, a third batch of V₃O₇H₂O was prepared without using GO (oven dried), a fourth batch was prepared with GO and poly ethylene oxide (1% PEO in water was added to the GO suspension, then spray-dried and heat-treated at 400° C. for 2 hours), and a fifth batch was prepared with PEO (1% in water, but without GO) via spray-drying, followed by heat-treating at 400° C. for 2 hours. Heat treatment of PEO at 400° C. serves to convert PEO to a carbon material. The particulates of GO/V₃O₇H₂O composite were used as a cathode active material in a Li metal cell.

Example 8 Preparation of Electrodes for Li-Ion Cells

Several dry electrodes containing graphene-enhanced particulates (e.g. comprising lithium cobalt oxide or lithium iron phosphate primary particles embraced by graphene sheets) were prepared by mixing the particulates with a liquid to form a paste without using a binder such as PVDF. The paste was cast onto a surface of a piece of glass, with the liquid medium removed to obtain a dry electrode. Another dry electrode was prepared by directly mixing LiFePO₄ primary particles with graphene sheets in an identical liquid to form a paste without using a binder. Again, the paste was then cast to form a dry electrode. The dry electrodes were for the evaluation of the effect of various conductive additives on the electrical conductivity of an electrode.

For comparison purposes, several additional dried electrodes were prepared under exactly identical conditions, and the paste in each case was made to contain the same cathode active particles, but a comparable amount of other conductive additives: multi-walled carbon nano-tubes (CNTs), carbon black (Super-P from Timcal), a CNT/Super-P mixture at an 1/1 ratio, and a GO/Super-P mixture at an 1/1 ratio. Corresponding “wet” electrodes for incorporation in a battery cell were made to contain a PVDF binder. These electrodes were made into full cells containing graphite particles or lithium metal as an anode active material.

The first-cycle discharge capacity data of small full button cells containing lithium metal as an anode active material were obtained. The data show that the battery cells containing graphene-enhanced particulates in the cathode show superior rate capability to that of a carbon black-enhanced cathode. Most importantly, the Li-ion cells having a higher salt concentration in an ionic liquid solvent typically exhibit a longer and more stable cycling life, experiencing a significantly lesser extent of capacity decay after a give number of charge/discharge cycles.

It may be further noted that the cathode active material that can be used in the presently invented electrode is not limited to lithium cobalt oxide and lithium iron phosphate. There is no particular limitation on the type of electrode active materials that can be used.

Example 9 Li-Air Cells with Ionic Liquid Electrolytes Containing Various Salt Concentrations

To test the performance of the Li-air battery employing room temperature ionic liquid with different lithium salt concentrations, several pouch cells with dimension of 5 cm×5 cm were built. Porous carbon electrodes were prepared by first preparing ink slurries by dissolving a 90 wt % EC600JD Ketjen black (AkzoNobel) and 5 wt. % Kynar PVDF (Arkema Corporation) in Nmethyl-2-pyrrolidone (NMP). Air electrodes were prepared with a carbon loading of approximately 20.0 mg/cm² by hand-painting the inks onto a carbon cloth (PANEX 35, Zoltek Corporation), which was then dried at 180° C. overnight. The total geometric area of the electrodes was 3.93 cm². The Li/O² test pouch cells were assembled in an argon-filled glove box. The cell consists of metallic lithium anode and the air electrode as a cathode, prepared as mentioned above. The copper current collector for anode and the aluminum current collector for cathode were used. A Celgard 3401 separator separating the two electrodes was soaked in EMITFSI/LiTFSI solutions (different concentrations) for a minimum of 24 hours. The cathode was soaked in the oxygen saturated EMITFSI/LiTFSI solution for 24 hours and was placed under vacuum for an hour before being used for the cell assembly. The cell was placed in an oxygen filled glove box where oxygen pressure was maintained at 1 atm. Cell charge-discharge was carried out with a battery cycler at the current rate of 0.1 mA/cm² at room temperature. It was found that a higher lithium salt concentration in an ionic liquid results in a higher round-trip efficiency for cells (63%, 67%, and 74% for 1.0 M, 3.0 M, and 5.5 M, respectively) and lower capacity decay after a given number of charge/discharge cycles (21%, 7%, and 4.5% for cells with 1.0 M, 3.0 M, and 5.5 M, respectively, after 100 cycles).

Example 10 Evaluation of Electrochemical Performance of Various Cells

Charge storage capacities were measured periodically and recorded as a function of the number of cycles. The specific discharge capacity herein referred to is the total charge inserted into the cathode during the discharge, per unit mass of the composite cathode (counting the weights of cathode active material, conductive additive or support, binder, and any optional additive combined). The specific charge capacity refers to the amount of charges per unit mass of the composite cathode. The specific energy and specific power values presented in this section are based on the total cell weight. The morphological or micro-structural changes of selected samples after a desired number of repeated charging and recharging cycles were observed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

As an example, the cycling performance (charge specific capacity, discharge specific capacity, and Coulomb efficiency) of a Li metal-sulfur cell containing a low-concentration electrolyte (1 M of lithium salt in an ionic liquid) is shown in FIG. 3(A). Some representative charge-discharge curves of the same cell are presented in FIG. 3(B). It is quite clear that the capacity of the cell rapidly decays as charges and discharges are repeated. This is characteristic of conventional Li—S cells that have great propensity for sulfur and lithium polysulfide to get dissolved in the electrolyte at the cathode side. Much of the dissolved sulfur could not be re-deposited to the cathode conductive additive/substrate or the cathode current collector during subsequent charges/discharges. Most critically, as time goes on or when charge/discharge cycling continues, some of the dissolved lithium polysulfide species migrate to the anode side and react with Li to form insoluble products and, hence, these species could not return to the cathode. These lead to continuing decay in the battery capacity.

We proceeded to investigate how the lithium salt concentration would affect the lithium polysulfide dissolution in an ionic liquid solvent, as opposed to an organic solvent, and to determine how concentration changes would impact the thermodynamics and kinetics of the shuttling effect. We immediately encounter some major challenges. First, we did not have a wide range of lithium salt concentrations at our disposal. Most of the lithium salts could not be dissolved in those solvents commonly used in Li-ion or Li—S secondary cells for more than 2 moles/liter (M). As a matter of fact, open literature reports and patent documents indicate that the electrolyte solute concentrations in an ionic liquid solvent in the majority of the Li-ion or Li—S secondary cells are in the range of 0.5 M to 1.5 M.

Second, we quickly came to realize that the viscosity of all room temperature ionic liquids (RTILs) was already extremely high at room temperature and adding more than 1.5 M of lithium salt in such a viscous solid made the resulting mixture look like and behave like a solid. It was next to impossible to use a stirrer to help disperse the solid lithium salt powder in the ionic liquid solvent. Further, a higher solute concentration was (still is) generally believed to be undesirable since a higher concentration normally results in a lower lithium ion conductivity in the electrolyte. This is not conducive to achieving a higher power density, lower polarization, and higher energy density (at high charge/discharge rates). We almost gave up, but decided to move forward anyway. The research results have been most surprising.

Contrary to the expectations by electrochemists and battery designers hat a significantly higher lithium salt concentration could not be produced, we found that a concentration as high as 3-11 M of a lithium salt in an ionic liquid could be achieved. For instance, >8.0 M of LiTFSI salt could be dissolved in an ionic liquid (e.g. EMITFSI) containing a small amount of a solubility-promoting agent (DOL) with an EMITFSI-to-DOL ratio of 9:1. Once a complete dissolution of a lithium salt in a mixture solvent is attained, we could choose to selectively remove the solubility-promoting agent (typically a more volatile organic solvent). We were pleasantly surprised to observe that partial or complete removal of the solubility-promoting agent upon complete salt dissolution would not result in crystallization or precipitation of the salt from the ionic liquid solvent even though the salt (a solute) is in a highly supersaturated condition.

We have further defied the expectation of battery chemists and engineers that a higher electrolyte concentration would lead to a lower discharge capacity. Most surprisingly, the Li—S cells contain a higher-concentration electrolyte system exhibit not only a generally higher energy density but also a dramatically more stable cycling behavior and longer cycle life.

As an example, FIG. 4(A) shows the charge specific capacity, discharge specific capacity, and Coulomb efficiency of a Li metal-sulfur cell containing an IL electrolyte (C=3.5 M). The cycling performance is so much better than that of the corresponding cell having a lower salt concentration as shown in FIGS. 3(A) and 3(B). The specific capacity of this lower concentration cell decays rapidly as the number of charge/discharge cycles increases. As shown in FIG. 4(B), the behaviors of a corresponding cell with an even higher-concentration electrolyte (5.5 M) are further better than those of the cell with C=3.5 M.

FIG. 5 shows the Ragone plots (cell power density vs. cell energy density) of three Li metal-sulfur cells each having an exfoliated graphite worm-sulfur cathode, but the lithium salt concentrations being 1.0 M, 3.5 M, and 5.5 M, respectively. Not only the energy density, but also the power density of a Li—S cell is improved when the ionic liquid-based solvent has a higher lithium salt concentration. This is completely opposite to the expectations of electrochemists and battery designers that (1) ionic liquids are already thick and viscous and should not be capable of dissolving more than 2 M of lithium salt; and (2) with a higher salt concentration, the electrolyte viscosity should be even higher, making the lithium ions even less mobile with a lower diffusion coefficient and, hence, leading to a reduced lithium ion migration and reduced power density. A logical question to ask is why a higher electrolyte concentration (higher than 3 M) seems to allow for a facile transport of lithium ions, Li⁺.

When lithium ions are formed in the conventional lower-concentration electrolyte, the positively charged lithium ions Li⁺ might be associated with or surrounded by the solvating anions that help to dissolve or “solvate” the Li⁺ ions. Typically, one Li⁺ ion can be clustered with several (2-4) solvating anions. In other words, when a Li⁺ ion moves, it has to drag along several anions to move with it. Such a Li⁺ ion transport mechanism would be very sensitive to the variation in electrolyte viscosity, which would increase with increasing lithium salt concentration and decreasing temperature. In contrast, with a much higher lithium salt concentration, there would be significantly more Li⁺ ions than the required solvating anions in the electrolyte. Consequently, many of the Li⁺ ions become “free” ions when the lithium salt concentration is sufficiently high. These free Li⁺ ions could move faster than if they were clustered with solvating anions as in the electrolyte of a low salt concentration. It seems that when the lithium salt concentration is greater than 3 M, the free Li⁺ ions would significantly outnumber the solvated Li⁺ ions, and the number of free Li⁺ ions would also be greater than the total number of Li⁺ ions (regardless if they are clustered with solvating anions or not) in an electrolyte having a concentration <1 M.

Additional data on the correlation between electrochemical performance of various Li metal cells and Li-ion cells are presented in Table 1 below.

TABLE 1 Examples of the electrolytes and lithium salt concentrations used. Cathode IL Solvent + Capacity active solubility- Anode Cathode loss material Cell promoting C active active after 50 utilization ID agent Li salt (solute) (M) material material cycles rate G-1 PP₁₃TFSI + (LiN(CF₃SO₂)₂ 1.0 Li metal 50% S + 61% 55% G-2 DOL (ratio = 2.0 50% CB 42% 56% G-3 7:1) 6.0 2.5% 63% G-4 8.0 0% 66% H-1 PP₁₃TFSI + (LiN(CF₃SO₂)₂ 1.5 Li metal 80% S + 9% 81% H-2 TEGDME 5.5 20% EG 1.0% 88% H-3 (ratio = 6:1) 7.2 0% 92% H-4 9.6 0% 93% J-1 DEMETFSI LiCF₃SO₃ 1.0 Li metal 80% 11% 68% J-2 4.5 Li_(x)V₃O₈ 2.1% 78% J-3 6.5 sheets + 0.3% 83% 20% CNT L-1 DEMETFSI LiBOB 2.5 Lithiated 80% TiS₂ + 4.5% 82% L-2 3.5 Si nano 20% EG 2.2% 87% L-3 4.5 particles 1.6% 89% L-4 5.5 1.0% 92% M-1 BEPyTFSI + LiTFSI 1.0 Li metal 80% S + 11% 75% M-2 DMC (ratio = 4.0 20% NGP 3.5% 84% 10:1)

Based on the data as summarized in Table 1, one can make the following significant observations:

-   -   (a) The higher-concentration electrolyte results in a         dramatically better battery cycling performance (higher capacity         retention rate or lower capacity loss) and a higher cathode         active material utilization rate, as compared to the         lower-concentration electrolyte.     -   (b) In addition to the implementation of a higher concentration         electrolyte system, a meso-porous or porous nano-structured         cathode supporting network composed of exfoliation graphite         worms (EGW) and conductive nano-filaments, such as carbon         nanotubes (CNTs) and graphene (NGPs), further improves the         cycling stability and active material utilization rate in a         rechargeable Li—S cell. Among these conductive network         structures for sulfur and/or lithium polysulfide, AEGWs appear         to provide the best overall performance.

Not wishing to be bound by theory, but we believe that the high-concentration electrolyte is capable of inhibiting lithium polysulfide dissolution, thus overcoming the polysulfide shuttle phenomenon. Consequently, a Coulombic efficiency nearing 100% and long cycle life can be achieved. The solubility of lithium polysulfide (ξ) is affected by the concentration of lithium ions already present in the electrolyte by the common ion effect. The solubility product (Ksp) of lithium polysulfide may be written as:

Li₂S_(n)⇄2Li⁺+S_(n) ²⁻;K_(sp)=[Li⁺]²[S_(n) ²⁻]=4ξ_(o) ³;ξ_(o)=(K_(sp)/4)^(1/3)  (Eq.1),

where ξ_(o) represents the solubility of lithium polysulfide when no lithium ion is present in the solvent. If the concentration of the lithium salt in the electrolyte (C) is significantly larger than the solubility of polysulfide, the solubility of polysulfide in the electrolyte containing the concentrated lithium salt can be expressed as:

ξ/ξ_(o)=(2ξ_(o)/C)²  (Eq.2).

Therefore, when a concentrated electrolyte is used, the solubility of lithium polysulfide will be reduced significantly. In other words, few lithium polysulfide ions will get dissolved. Furthermore, the high salt concentration in the electrolyte is also less amenable to the diffusion of large chemical species, such as lithium polysulfide, from the cathode to the anode side. These would reduce the shuttling effect.

In summary, the present invention provides an innovative, versatile, and surprisingly effective platform materials technology that enables the design and manufacture of superior lithium metal and lithium-ion rechargeable batteries. The lithium cell featuring a high-concentration electrolyte system exhibits a stable and safe anode (no dendrite-like feature), high lithium utilization rate, high cathode active material utilization rate, high specific capacity, high specific energy, high power density, little or no shuttling effect, and long cycle life.

We have further observed that the electrochemical performance of lithium secondary cells containing, as electrolyte, a high concentration of lithium salt dissolved in an ionic liquid solvent is relatively independent of the battery operating temperature and the cells can effectively operate in an unusually wide temperature range, extending to a very low temperature and a very high temperature regime. This temperature range is the widest among all known lithium batteries.

The presently invented cells can provide a specific energy greater than 400 Wh/Kg (more typically greater than 600 Wh/Kg, often greater than 800 Wh/Kg, and even achieving an unprecedented 1,000 Wh/Kg) based on the total cell weight including anode, cathode, electrolyte, separator, and current collector weights combined. This has not been achieved by any prior art approaches. 

1. A rechargeable lithium metal or lithium-ion cell comprising a cathode having a cathode active material or conductive supporting structure, an anode having an anode active material, a porous separator electronically separating said anode and said cathode, a highly concentrated electrolyte in contact with said cathode active material or conductive supporting structure and said anode active material, wherein said electrolyte contains a lithium salt dissolved in a first ionic liquid solvent with a concentration greater than 3 M.
 2. The rechargeable lithium metal or lithium-ion cell of claim 1, which is a lithium-sulfur cell or lithium-ion sulfur cell containing a sulfur cathode.
 3. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said concentration is greater than 4 M.
 4. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said concentration is greater than 5 M.
 5. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said concentration is greater than 7 M.
 6. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said concentration is greater than 10 M.
 7. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said first ionic liquid solvent is a room temperature ionic liquid.
 8. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said first ionic liquid solvent is selected from a room temperature ionic liquid having a cation selected from tetraalkylammonium, di-, tri-, or tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, trialkylsulfonium, or a combination thereof.
 9. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said first ionic liquid solvent is selected from a room temperature ionic liquid having an anion selected from BF₄ ⁻, B(CN)₄ ⁻, CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆ ⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻, N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻ or a combination thereof.
 10. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains a solubility-promoting agent.
 11. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains a solubility-promoting agent and a ratio of the solubility-promoting agent amount to the first ionic liquid amount is less than 1/3.
 12. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains a diluent and a ratio of the solubility-promoting agent amount to the first ionic liquid amount is less than 1/5.
 13. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains a solubility-promoting agent and a ratio of the solubility-promoting agent amount to the first ionic liquid amount is less than 1/9.
 14. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains a solubility-promoting agent selected from an organic liquid or a second ionic liquid.
 15. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains an ether-based solubility-promoting agent selected from 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, a combination thereof, or a combination with a carbonate.
 16. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said electrolyte further contains an carbonate-based solubility-promoting agent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene or methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), or a combination thereof.
 17. The rechargeable lithium metal or lithium-ion cell of claim 1 wherein said lithium salt is selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-metasulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂, lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithium bisperfluoroethysulfonylimide (LiBETI), an ionic liquid lithium salt, or a combination thereof.
 18. The rechargeable lithium metal or lithium-ion cell of claim 1 wherein said first ionic liquid is further mixed with an organic solvent-based solubility-promoting agent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene or methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), Poly(ethylene glycol)dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, or a combination thereof, and wherein said first ionic liquid solvent-to-organic solvent ratio is no less than 10:1.
 19. The rechargeable lithium metal or lithium-ion cell of claim 1 wherein said anode contains an anode active material selected from lithium metal, a lithium metal alloy, a mixture of lithium metal or lithium alloy with a lithium intercalation compound, a lithiated compound, lithiated titanium dioxide, lithium titanate, lithium manganate, a lithium transition metal oxide, Li₄Ti₅O₁₂, or a combination thereof.
 20. The rechargeable lithium metal or lithium-ion cell of claim 1 wherein said cell is a lithium-ion cell or a lithium ion-sulfur cell and said anode contains an anode active material selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd), and lithiated versions thereof; (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements, and lithiated versions thereof, wherein said alloys or compounds are stoichiometric or non-stoichiometric; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites, and lithiated versions thereof; (d) salts and hydroxides of Sn and lithiated versions thereof; (e) carbon or graphite materials and prelithiated versions thereof; and combinations thereof.
 21. The rechargeable lithium metal or lithium-ion cell of claim 1 wherein said cathode comprises: (a) an integrated nano-structure of electrically conductive nanometer-scaled filaments that are interconnected to form a porous network of electron-conducting paths comprising pores with a size smaller than 100 nm, wherein said filaments have a transverse dimension less than 500 nm; and (b) powder or coating of said cathode active material disposed in said pores or deposited on a nano-scaled filament surface, wherein said cathode active material is in contact with said electrolyte.
 22. The rechargeable lithium metal or lithium-ion cell of claim 21 wherein said filaments comprise an electrically conductive material selected from the group consisting of electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, carbon nanowires, nano graphene sheets or platelets, metal nano wires, metal-coated nanowires or nano-fibers, conductive polymer-coated nanowires or nano-fibers, and combinations thereof.
 23. The rechargeable lithium metal or lithium-ion cell of claim 21 wherein said filaments comprise nano graphene sheets or platelets selected from single-layer or few-layer pristine graphene, reduced graphene oxide, graphene oxide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof, wherein few-layer is defined as 10 planes of hexagonal carbon atoms or less.
 24. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said anode contains a nano-structure comprising an integrated structure of electrically conductive nanometer-scaled filaments that are interconnected to form a porous network of electron-conducting paths comprising interconnected pores, wherein said filaments have a transverse dimension less than 500 nm and wherein said nano-structured anode provides a sufficient amount of nano-filament surface areas to support lithium metal when the cell is in a charged state.
 25. The rechargeable lithium metal or lithium-ion cell of claim 1 wherein said cathode has an active material utilization rate no less than 90%.
 26. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said cathode active material is selected from a transition metal dichalcogenide, a transition metal trichalcogenide, a transition metal oxide, or a combination thereof.
 27. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said cathode active material is selected from TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂, an iron oxide, a vanadium oxide, or a combination thereof.
 28. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said cathode active material contains a vanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂, V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.
 29. The rechargeable lithium metal or lithium-ion cell of claim 1, wherein said cathode active material contains a layered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.
 30. The rechargeable lithium metal or lithium-ion cell of claim 1, which is a Lithium-air cell having a higher round-trip efficiency or higher resistance to capacity decay as compared to a corresponding Lithium-air cell that has an electrolyte salt concentration lower than 3M.
 31. The rechargeable lithium metal or lithium-ion cell of claim 1, further comprising a layer of protective material disposed between the anode and the electrolyte wherein the protective material is a lithium ion conductor. 